1. Field of the Invention
Embodiments of the invention relate generally to the field of measurement apparatus and methods. More particularly, embodiments of the invention relate to the apparatus and methods of scanning probe microscopy.
2. Discussion of the Related Art
Prior art scanning probe microscopy apparatus and methods are known to those skilled in the art. For instance, a conventional scanning probe microscope is shown in FIG. 1. A sample 100 is placed on a mount and a cantilever sensor 101 with a sharp tip is brought close to the sample surface. Interactions between the sample surface and the sensor tip lead to flexural and torsional deflections of the cantilever, the magnitude of which can vary from sub-nanometer to hundred nanometer range depending on operation mode. A laser 102 is deflected off the top of the cantilever into a detector 103, which is connected to driving circuitry 104. Other realizations of deflection sensors based on piezoresistive, piezoelectric, capacitive, MOSFET, tuning fork, double tuning fork, and other position sensors are well known. Current flowing through the cantilever or cantilever-surface capacitance can be other modes of detection. Typically, the sample surface is scanned point by point to obtain the topography or functional properties of the surface. Alternatively, the response is measured in a single point as a function of probe-surface separation, probe bias, etc., constituting spectroscopic modes of operation.
A problem with this technology has been that methods existing to date are generally based on the detection of the signal under a constant excitation or at a periodic excitation at a single frequency. In the constant or static mode, static tip deflection (or other static parameter such as tip-surface dc current) is used to serve as a feedback signal to maintain constant tip-surface separation or property measurement. In the periodic excitation mode, the amplitude or phase of cantilever oscillations or other oscillatory response is selected using lock-in amplifier or similar circuit and used as a feedback or detection signal. In the frequency tracking modes, the cantilever or other sensor is kept at a corresponding mechanical resonance, and changes in the dynamic characteristics of oscillation (e.g. resonant frequency or amplitude at the resonance) are detected and used as feedback or detected signals. All these modes severely limit the amount of information obtainable by the scanning probe microscope. Therefore, what is required is a solution that provides maximum information about the tip-surface interactions.
A number of SPM techniques (e.g. Force Volume Imaging, Pulsed Force Mode and Molecular Recognition Mode) are based on the specially designed large-amplitude waveforms that probe different parts of the force-distance curve to distinguish short- and long range interactions. These methods also have similar limitations, since either static force (force-distance measurements per se) or response at single frequency is measured at different positions of the probe tip with respect to the surface.
The fundamental problem with this technology has been that the resonance frequency, amplitude and quality factor (Q-factor) of the cantilever vibrating in contact with the surface under constant mechanical excitation, the three parameters that provide the complete description of the system in the simple-harmonic oscillator approximation, cannot be unambiguously separated. Therefore, what is also required is a solution that allows separation of these parameters.
One unsatisfactory approach, in an attempt to solve the above-discussed problems involves sweeping the excitation frequency at each sample point. However, a disadvantage of this approach is the significant time (1-10 s) per point, leading to unreasonable data acquisition times. A 100×100 pixel requires a time on the order of 10 s of hours.
Heretofore, the requirements of maximizing information about tip-surface interactions and obtaining (a) independent amplitude, resonant frequency and Q-factor parameters and (b) characterization of the complete behavior of the system referred to above have not been fully met. What is needed is a solution that solves all of these problems, preferably simultaneously.